1. Field of the Invention
This invention relates to a drive and bearing mechanism to rotate components in a spacecraft and, more particularly, to a solar array drive having a drive shaft rotated precisely and rotatably supported for high shaft stiffness under varying temperature conditions in a space environment.
2. Description of the Prior Art
In structures having an element which is to be rotated very precisely it is required to rotatably support such a member to allow as little free play in the axial and radial directions as possible and minimum bending under application of loads. Rolling-contact bearings, having race supported spherical balls, are commonly used to mount a rotating member, and certain types of bearings when preloaded in an axial direction are known and used to provide the axial and radial constraint of the rotating member. A difficult problem arises in a zero free-play mechanism when the components of such a mechanism are composed of different materials having different temperature coefficients and subjected to varying temperature conditions. Under temperature changes, severe stresses can be induced as a result of different thermal expansion properties of the components, causing loss of precision or a possible failure of the mechanism.
Some prior art devices incorporate helical spring means with rolling-contact bearings to resiliently take up the bearing looseness. Such devices allow for an axially sliding movement of an axially pressed bearing race with respect to the rotating member or housing on which it is mounted. This axial sliding movement can result in a reduction or loss of axial or radial constraints.
United States Pat. No. 2,885,583, issued on May 5, 1959, discloses a rotating structure having a disc spring allowing for expansion and contraction of the operating parts under high temperature and low pressure. The disc spring biases a bearing race axially to restrict the movement of the balls to a single annular path, but allows for movement of the race in a free floating relationship when the forces due to thermal deflections exceed the forces of the disc spring.
In a spacecraft or satellite which utilizes solar array panels for converting solar energy into electrical energy for its power needs, the solar arrays are usually rotated to keep the arrays facing the sun. Maximum exposure requires a precise rotation of the panel to face normal to the sun's rays. A solar array drive coupled to a rotating drive shaft is generally used to rotate the solar arrays while the spacecraft is in orbit. The stiffness of the solar array drive shaft is a critical factor in the attitude control and stability of the spacecraft since the shaft is an integral structural member of the solar array support. Precise rotation of the shaft is also required, to prevent undesirable vibrations which will affect the attitude of the spacecraft and unduly stress the components in the solar array drive.
Achieving high shaft stiffness as well as precision rotation of the shaft in solar array drives has presented difficult shaft mounting problems, especially in spacecraft designed for long-life capability. In particular, rolling-contact bearings used to rotatably support the shaft are often unduly overloaded in an attempt to increase the moment stiffness of the shaft against the forces imposed upon the shaft by the solar panels. Such overloaded bearings result in a reduction of the life of the bearings and hence the life of the spacecraft. Providing adequate thermal compensation for thermal deflections resulting from the differences in thermal expansion properties of the component materials has often required a sacrifice of the shaft stiffness. Such a trade-off of either the shaft stiffness or thermal expansion compensation undesirably reduces the capability of the device and, under severe operating conditions, the life of the device will be curtailed.